This invention relates generally to a quartz analog movement for timepieces of the type utilizing a Lavet stepping motor, and more particularly it relates to an improved method for manufacturing the stepping motor for a wrist watch.
A stepping motor of the Lavet type consists of a coil winding with a core member of magnetizable material and a stator forming a magnetic path for the flux and terminating in two semi-circular pole shoes which cooperate with a rotor having a permanent magnet. Periodic pulses of current to the coil step the rotor which turns the hands of the wrist watch through a gear train. Originally the stators were made in two pieces separated by a narrow air gap, together with notches or cut-outs in the pole shoes which would cause the rotor to assume a preferred rest position between steps. Because of the small size of a wrist watch and very close clearances required, it was very difficult to properly position the stator members with respect to one another and with respect to the rotor. For example, U.S. Pat. No. 3,949,251-Takatsuki (Daini Seiko) describes a stepping motor with off-set stator halves which are positioned by pivoting them against a positioning ring. Another construction is shown in U.S. Pat. No. 3,978,651-Yoshino (Suwa Seiko) describes a two piece stator construction for a stepping motor using off-set stator pieces which are attached to a coil bobbin and core piece. The two stator halves are pivotably mounted on a reference plate. The reference plate is then affixed to the watch frame after the stator parts are assembled to it. This patent suggests that the stator plate might be mounted to the reference plate as a single one-piece stator and then cut apart to form a two-piece stator.
Other prior art patents showing two-piece stepping motors are seen in U.S. Pat. Nos. 3,984,972-Yoshino and 2,704,334-Brailsford.
Because of the difficulties in getting the two stator halves into proper position in the watch frame, one-piece stators were suggested. One of these is seen in U.S. Pat. No. 4,398,107-Fukushima (Daini Seiko) with a region of reduced cross sectional area which magnetically saturates in lieu of an air gap. While this reduces the number of parts, the performance of the motor is dependent upon manufacturing accuracy in producing the regions of reduced cross sectional area, sometimes requiring special materials or increased manufacturing costs. For example, in U.S. Pat. No. 4,361,772 -Fukushima, a stator piece of low saturated magnetic flux density must be used in order not to require too high a percentage of magnetic flux (with consequent loss) to saturate the regions of reduced cross-sectional area.
It would be desirable to provide the manufacturing convenience of a one-piece stator which is combined with the improved operating performance to be expected with a two-piece stator.
Accordingly, one object of the present invention is to provide an improved manufacturing method for a timepiece stepping motor which reduces its cost.
Another object of the invention is to provide an improved timepiece movement and method of manufacture which will reduce the cost of the movement.